The present invention is related to polyurethanes, and particularly to urea-modified polyurethanes.
The chemistry of urethane polymers extends over a very broad scope of chemical structures, producing products varying widely in properties and uses. A common variation from the basic urethane backbone structure (i.e.. the carbamate linkage) is the incorporation of urea linkages, which is generally accomplished by the use of diamines as chain extenders.
Among the many uses of polyurethanes are those involving medical devices, where polyurethanes are used as structural materials for separatory devices and prosthetic devices, and also as adhesives in such devices. The polyurethanes assume a variety of forms, in these materials, including sheets, tubing, structural components, and two-component adhesive composition. In many cases, the polymer is in intimate contact with internal human tissue. In such applications, compatibility with the tissue is essential, and certain types of polyurethanes, notably those in which the polyol component is castor oil or various polyols similar in structure and properties, have been found to provide biocompatible products.
The various polyurethanes and their components have certain disadvantages which affect their utility, particularly in medical use. Biocompatibility, as mentioned above, is a problem in many cases. In addition, the product color is important, particularly when the product must be observed during use. This may occur for example when one is monitoring a clinical treatment process or a patient's progress, where a high degree of clarity and transparency is desired. In two-component polyurethane systems, the stability of each of the components and possible dangers to those handling them (such as toxicity considerations, particularly carcogenicity) are a concern. Still further, certain combinations of diamines and polyols have a strong tendency to react with each other, thereby interfering with the formation of the carbamate groups in the basic polyurethane curing reaction, which not only interferes with the polyurethane formation but results in a loss of the chain-extending function of the diamine.